Variable displacement pumps generally include a plurality of pistons held against the driving surface of a tiltable swashplate. A joint such as a ball and socket joint is disposed between each piston and the swashplate to allow for relative movement between the swashplate and the pistons. Each piston is slidably disposed to reciprocate within an associated barrel as the pistons rotate relative to the tilted surface of the swashplate. As each piston is retracted from the associated barrel, low pressure fluid is drawn into that barrel. When the piston is forced back into the barrel by the driving surface of the swashplate, the piston pushes the fluid from the barrel at an elevated pressure.
The tilt angle of the swashplate is directly related to an amount of fluid pushed from each barrel during a single relative rotation between the pistons and the swashplate. And, based on a restriction of the pump and/or a fluid circuit connected to the pump, the amount of fluid pushed from the barrel during each rotation is directly related to the flow rate and pressure of fluid exiting the pump. Thus, a higher tilt angle equates to a greater flow rate and pressure, while a lower tilt angle results in a lower flow rate and pressure. Similarly, a higher tilt angle requires more power from a driving source to produce the higher flow rates and pressures than does a lower tilt angle. As such, when the demand for fluid is low, the swashplate angle is typically reduced to lower the power consumption of the pump.
Some variable displacement pumps utilize a hydraulic piston to adjust the tilt angle of the swashplate. The hydraulic piston is connected to the swashplate and moved by an imbalance of forces on the hydraulic piston. As the hydraulic piston is moved to retract or extend, the tilt angle of the swashplate is increased or decreased. Movement of the hydraulic piston is generally controlled by way of a pilot or solenoid activated linear spool valve. Unfortunately, linear spool valves may lack accuracy in their control over movement of the hydraulic piston.
One attempt at improving control accuracy of the hydraulic piston is described in U.S. Pat. No. 4,205,590 (the '590 patent) issued to Stegner on Jun. 3, 1980. The '590 patent discloses a pump having a stationary housing surrounding a rotatable cylinder block adapted to be rotated by a shaft. The block has a pair of pump pistons arranged against a swashplate on opposite sides of the shaft. A fluid actuator is provided to control displacement of the pump pistons, and includes a first control piston and a second control piston linked to the swashplate. By regulating a flow of fluid to the control pistons, the control pistons can pivot the swashplate and thereby control a stroke length of each of the pump pistons.
The pump of the '590 patent also includes a lobed cylindrical valve spool, and a sleeve member that receives the valve spool. A polarized torque motor is connected to linearly move the valve spool relative to the sleeve by way of a flapper. When the flapper is moved by the motor in a first direction, the valve spool is urged to communicate pressurized fluid with the first control piston and drain the second control piston of fluid, thereby tilting the swashplate to increase a displacement of the pump pistons. When the flapper is moved by the motor in a second direction, the valve spool is urged to communicate pressurized fluid with the second control piston and drain the first control piston of fluid, thereby tilting the swashplate to decrease a displacement of the pump pistons. A return spring is situated to return the flapper to a neutral position. In this configuration, a force applied by the motor to the flapper will move the valve spool until a resistance of the return spring balances the force of the motor.
An articulated feedback mechanism is connected between the swashplate and the sleeve member of the '590 patent. As the swashplate tilts, the articulated feedback mechanism pivots and transfers the tilting motion to a linear motion of the sleeve member relative to the spool valve. When a desired angle of the swashplate is achieved, the sleeve member is sufficiently moved by the tilting motion to block fluid flow through the valve spool so as to maintain the desired angle.
In the pump configuration of the '590 patent, a change in current input to the torque motor will produce a proportional change in the valve spool position. A change in valve spool position will produce a change in the positions of the pump pistons, thereby changing the angularity of the swashplate about its tilt axis. The feedback mechanism moves through the same angle as the swashplate and, by its articulated connection with the sleeve member, slaves the sleeve member on the valve spool. Thus, a current applied to the motor will be proportional to a displacement of the pump.
Although the pump of the '590 patent may improve displacement control accuracy, it may still be limited. Specifically, the pump relies on precise control over the current applied to the motor in order to ensure displacement accuracy. If the applied current varies, the actual displacement may not match a desired displacement. And, as the components of the pump age or the pump is used in varying environments and for different applications, precise control over the current may be difficult to ensure. In addition, the flapper configuration may have low durability and responsiveness. Further, the linear translation of the valve spool may create dynamic fluid interactions that could reduce displacement accuracy of the pump.
The axial piston device of the present disclosure solves one or more of the problems set forth above and/or other problems.